Cryogenic air separation is a very energy intensive process due to the need to generate very low temperature refrigeration and separate feed constituents of low relative volatility. The cryogenic air separation process is further complicated when it is integrated with a liquefaction process to recover substantial flows of liquid products from the air separation unit. In cryogenic air separation units designed to produce a large amount of liquid products, such as liquid oxygen, liquid nitrogen and liquid argon, a large amount of refrigeration must be provided, typically through the use of multi-turbine process arrangements.
A broad set of refrigeration configurations are designed to expand the feed air. Feed air expansion arrangements are often referred to as air pre-expansion configurations. High pressure feed air may be first cooled and then expanded in whole or in part to any one of the nitrogen rectification sections of the column system. In many instances, the demand for liquid products eclipses the potential production from air pre-expansion. In such circumstances, a warm turbine may be configured to expand air or another fluid for purposes of warm end fore-cooling. Such arrangements can be configured as open or semi-closed recycle systems. Such configurations impart refrigeration to the cryogenic air distillation column system via indirect heat exchange with the pre-purified, compressed feed air in the primary heat exchanger or in an auxiliary heat exchanger.
In the air pre-expansion arrangement, a portion of the pre-purified, compressed feed air is often further compressed in a boosted air compressor, partially cooled in the primary heat exchanger, and then all or a portion of this further compressed, partially cooled stream is diverted to a turbine. The expanded gas stream or exhaust stream is then directed to the higher pressure column of a dual pressure cryogenic air distillation column system. In some air pre-expansion arrangements, a portion of the compressed and purified air is diverted to a turbine without further compression in a booster air compressor,
Alternatively, a portion of the pre-purified, compressed feed air is partially cooled in the primary heat exchanger; a portion of this partially cooled stream is diverted to a second turbo-expander. The expanded gas stream or exhaust stream may be optionally cooled via direct or indirect heat exchange and directed to into a lower pressure column in the a thermally linked dual pressure distillation column system such as a two-column or three column distillation column system of a cryogenic air separation unit. The turbo-expansion of various column feed streams serves to refrigerate the distillation process. The work of expansion provides the refrigeration necessary to offset warm end temperature loss, process heat leak and to generate liquid products. In general, when column feed streams are expanded prior to column entry the refrigeration generated is subsequently recouped by the warming of the various product streams. The indirect heat exchange of warming column products provides then necessary cooling of the various feed air streams prior to column entry.
In order to increase the fraction of liquefied products extracted from the column system to above approximately 40% of the incoming feed air, refrigeration must be imparted to the cold end of the primary heat exchanger. Prior art processes have addressed this need by recycling a portion of the cold turbo-expanded gas stream through the primary heat exchanger.
Prior art cryogenic air separation processes have dealt with this issue by further turbo-expand the portion of air recycled to the cold turbine in an air separation unit to pressures at or near ambient pressure, as disclosed in U.S. Pat. No. 5,157,926. Such an approach, however suffers due to increased costs required to handle the near ambient pressure stream in the primary heat exchanger. In addition, the warm expansion turbine is constrained to operate between the pressure of the lower column and near ambient pressure. In addition such processes substantially increase the pre-purification demands on the process.
Accordingly, there is a need to reduce the costs associated with high liquid make cryogenic air separation units while maintaining high thermodynamic efficiency of the integrated cryogenic air separation and liquefaction system. Such solutions must also maintain the simplicity, reliability and relatively low cost of the rotating machinery used in the cold and warm turbines as well as the associated booster compression.